Greater Invocation; 6th
You can use this invocation to summon a swarm, as the insect plague spell. You add your Charisma modifier to the Fortitude save DC to resist the swarm’s distraction ability. The attacks of any locust swarm summoned by this ability are treated as magic weapons for the purpose of overcoming damage reduction.

Source: Complete Arcane

Insect Plague

Conjuration (Summoning)Level: Clr 5, Drd 5Components: V, S, DFCasting Time: 1 roundRange: Long (400 ft. + 40 ft./level)Effect: One swarm of locusts per three levels, each of which must be adjacent to at least one other swarmDuration: 1 min./levelSaving Throw: NoneSpell Resistance: No

You summon a number of swarms of locusts (one per three levels, to a maximum of six swarms at 18th level). The swarms must be summoned so that each one is adjacent to at least one other swarm (that is, the swarms must fill one contiguous area). You may summon the locust swarms so that they share the area of other creatures. Each swarm attacks any creatures occupying its area. The swarms are stationary after being summoned, and won’t pursue creatures that flee.
See the Monster Manual for details on locust swarms.